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Saturday, January 5, 2013

Math Shortcuts

Multiplying two numbers between 10
and 20

16 × 12

It’s simple,
just add the last digit of 12, i.e., 2 to 16. So 16 + 2 = 18. Multiply 18 by 10
mentally which gives 180 and add the product of the last digits of the two
numbers, i.e., 6 × 2 = 12. So, the answer is 180 + 12 = 192. You should take a maximum of 5 seconds to
do it mentally.

Multiplying two numbers between 20
and 30.

24 × 23

It’s simple.
Just add the last digit of 23, i.e., 3 to 24. So 24 + 3 = 27. Multiply 27 by 20
mentally which gives 540 and add the product of the last digits of the two
numbers, i.e., 4 × 3 = 12. So the answer is 540 + 12 = 552.

Multiplying two numbers one of which
is between 10 and 20 and the other between 20 and 30.

24 × 18

It’s simple.
The complement of 8 is 2. So, just subtract the complement of 8 i.e., 2 from
24, so 24 – 2 = 22. Multiply 22 by 20 mentally which given 440 and add the
product of the last digits of the two numbers, i.e., 4 × -2 = -8. So, the
answer is 440 – 8 – 432.